free breathing mufflers for speed
- Thread starter jseng1
- Start date
- Status
- Joined
- Oct 19, 2005
- Messages
- 18,978
Played with the ehaust calculator to see it gives a sliding scale over a handfull of inches in lenghts and OD suggestions, at least for Norris D cam which I don't expect much from til over 5 grand. I no longer like the Supertrap as by time you add enough plates to feel some improvement its about as loud w/o it and the weight banging/bouncing on muffler end gets to be damaging to stock Commando headers and mounts. Moderns have used sliding lengths and valves to open close exhaust paths per rpm/throttle but don't see how to apply it to vintage clunkers. I'm gonna try a special end that baffles and sucks some at low rpm but flows like a rocket at hi rpms. Will see what happens mounting past Peel's 2-1 on stock Trixie Combat. Old hot rodders othen had cable worked muffler cut out valves.
- Joined
- Jun 30, 2012
- Messages
- 14,006
Dkt26 said:
The wave length is calculated by dividing the speed of sound by the frequency in whatever system of units you are using. So the tuned length is chosen to suit the rev range you normally use. I work on the figure of 6,300 RPM which is the manufacturers' spec for most large British twins of the 60s. If you get it wrong, within reason it won't matter. It simply moves the power band a bit. I suggest the cross sectional area of the outlet of the collector on a 2 into one exhaust should be the sum of the two cross sectional areas of the header pipes. Other wise you lose too much off the top of the rev range. I work on 7,000 RPM max.
The header pipes I use on my 2 into one exhaust pipe were originally intended to be used to make a 4 into one pipe for a Kawasaki 900 (Z1). They are very skinny - Their ID matches the ID of the commando exhaust stub. Big diameter exhausts were replaced by skinny ones in the Triumph race kit for the T100 in the early 50s, and used on all of their later road bikes after the first Bonnevilles. I read claims that fat exhausts work better on manxes than skinny ones. The fastest A grader manx in Victoria in the early 60s had a skinny exhaust.
P.S. The header pipes on my exhaust are 25 inch long, the tail pipe including muffler is 32 inch long. the collector is about 3 inch long. I've advanced the standard 850 cam by 12 degrees to compensate. It is too loud for road use.
2" is approximately equal to two @ 1 3/8"
For 1 3/8" , πr squared = 1.48 sq inches +- . So the area of two pipes = roughly 3 square inches. One 2 inch is 3.14 sq. Inches,a bit more than 2 @ 1 3/8". If you want to get really precise numbers, subtract wall thicknesses before doing the arithmetic, probably makes no noticeable dif tho.
Glen
For 1 3/8" , πr squared = 1.48 sq inches +- . So the area of two pipes = roughly 3 square inches. One 2 inch is 3.14 sq. Inches,a bit more than 2 @ 1 3/8". If you want to get really precise numbers, subtract wall thicknesses before doing the arithmetic, probably makes no noticeable dif tho.
Glen
- Joined
- Jun 30, 2012
- Messages
- 14,006
worntorn said:
The ID of the header pipes is 3.7 cm, the ID of the tail pipe is 4.7cm. The area is 3.1415 X (R squared) ?
Header pipes 2 X 3.1415 X (1.85 squared) = 21.6 sq cm.
Tail pipe 3.1415 (2.4 squared) = 18.1 sq cm
There is almost no reduction in cross sectional area at the collector. I've allowed 1 mm wall thickness for the header pipes, it is probably 1.5 or 2 mm.
- Joined
- Jun 30, 2012
- Messages
- 14,006
If you know what your bike does under certain circumstances, it is worth experimenting by cutting the downside of the collector back and welding on different diameter tail pipes until you get a substantial improvement. I tried using a tail pipe the same size as the header pipes and the bike almost stopped. The trouble is that you have two variables to play with - the other is exhaust cam timing. Whatever you end up with you will probably have to compensate for with the exhaust valve opening point. As I've sai, I advanced the standard 850 cam by 12 degrees. The ex opening is earlier than 90 degrees BBDC and it closes way too early. The inlet opens at about 65 BTDC which is also important. The pipe is too loud for road use, however I had to increase the overall gearing. I bought a combat cam - that should make more sense if advanced 6 degrees.
- Status
